属性错误：'NoneType'对象没有属性'_next'

Question

我知道在将实例方法或任何预设函数插入到None类型时会发生这些错误，但由于我正在实现单链表，我需要为最后一个节点分配None类型。我如何解决这个问题，因为我无法更改此处的输入值。

以下是我的代码——

class singlyLinked:
class _node:
def __init__(self, data, next):
self._data = data
self._next = next

def __init__(self):
self._head=None
self._tail=None
self._size=0

def __len__(self):
return self._size

def isEmpty(self):
return self._size==0

def first(self):
if self.isEmpty():
raise Error('Queue is Empty')
return self._head._data #front aligned with the head of list

def dequeue(self):
if self.isEmpty():
raise Error('Queue is Empty')
answer=self._head._data
self._head=self._head._next
self._size -= 1
if self.isEmpty():
self._tail=None
return answer

def enqueue(self, e):
newest=self._node(e, None)
if self.isEmpty():
self._head=newest
else:
self._tail._next=newest
self.tail=newest
self._size+=1

我在Jupyter笔记本中得到以下错误

AttributeError                            Traceback (most recent call last)
Cell In [20], line 3
1 a=singlyLinked()
2 a.enqueue(5)
----> 3 a.enqueue(5)
4 a.enqueue(5)
5 a.enqueue(5)
Cell In [19], line 41, in singlyLinked.enqueue(self, e)
39     self._head=newest
40 else:
---> 41     self._tail._next=newest
42 self.tail=newest
43 self._size+=1
AttributeError: 'NoneType' object has no attribute '_next'

Answer 1

您可以将入队函数更改为：

def enqueue(self, e):
newest=self._node(e, None)
if self.isEmpty():
self._head=newest
self._tail=newest
else:
self._tail._next=newest
self._tail=newest
self._size+=1

Answer 2

中有一个打字错误

newest=self._node(e, None)
if self.isEmpty():
self._head=newest
else:
self._tail._next=newest
self.tail=newest
self._size+=1

您需要：

self._tail = newest(带下划线(